


here, waiting for you

by EnRaa



Series: what kind of man...? [1]
Category: Undertale (Video Game)
Genre: Alternate Timelines, Angst and Fluff and Smut, Ecto-Genitalia (Undertale), Explicit Sexual Content, M/M, Timeline Shenanigans
Language: English
Status: Completed
Published: 2020-11-23
Updated: 2020-11-23
Packaged: 2021-03-09 19:21:25
Rating: Explicit
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 3,729
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/27681388
Author URL: https://archiveofourown.org/users/EnRaa/pseuds/EnRaa
Summary: He holds on, feeling for probably the first time in his life, and hopefully not the last, who Gaster is.
Relationships: W. D. Gaster/Sans
Series: what kind of man...? [1]
Series URL: https://archiveofourown.org/series/2025469
Comments: 11
Kudos: 29





	here, waiting for you

**Author's Note:**

> Hey, if ya'll want a really good song for Gaster/Sans timeline feels, check out "Love Psalm" by Mary Elizabeth McGlynn. It really sets up that angsty timeline goodness.

Sans knows who he is. Well, he knows _of_ him.

He doesn't know Gaster yet, in this timeline. He's heard the name revered in his classes, has read the headlines of his ground-breaking work bringing hope to the Underground, but he doesn't work with him yet. He's still a student, yet.

He hasn't relentlessly charmed and flirted his way into Gaster's affections, making sure the scientist seen no one but Sans and his stupidly beautiful grin.

So when Gaster sits down next to him and makes casual conversation, Sans is impossibly eager to get to know him.

Gaster knows he shouldn't do it, shouldn't confuse their fated meeting at the Royal Labs after Sans graduates like this, but finally being able to talk to Sans again, to actually interact with him and not have the conversation turn to _why did he do it_...?

This was safe, and he couldn't disappoint this Sans with his failures, so he leans in closer, sliding another drink to Sans as he listens to the smaller skeleton excitedly go into another theory.

He'd never known Sans when he was just a student. They didn't meet until Sans came to work for him at the labs, and he decides that he definitely missed out on knowing him when he was all unrestrained, chaotic knowledge that couldn't wait to test out all of his ideas. It's a familiar mindset that Gaster shares even now, and he wonders what caused Sans to lose that insatiable lust for knowledge.

For Gaster, his lust for knowledge left him cast off into the timelines, stripped from his home and likely never going to return, but Sans, to his knowledge, never had any sort of backlash to keep him from doing questionable things in the name of science.

It's an interesting development that has Gaster wondering if in another timeline, one where Sans didn't become more conscious of the dangers their work contained, there's a version of he and Gaster stuck outside of the timelines, blipping from one to the next, just like he is now, but _together_.

Sans is leaning closer now, his eyelights bright with excitement as he recounts his calculations, and Gaster barely notices how he reaches up to cup his mandible in his hand, gives him a reassuring smile when Sans falters, grin turning down at the corners ever so slightly until he realizes that Gaster isn't trying to shut him up, and then it's back full force as he leans into the touch.

The more Gaster questions him about his work, the more he lights up, and Gaster's made uncomfortably aware that this might be the first time Sans was able to really talk about his field without being shut down or having to dumb things down. It makes all of the late nights at the lab he and his Sans spent pouring over their work so much more special to him now, knowing he had this one thing with Sans no one else ever could.

They're pressed so close, and at some point it's just easier to scoop the smaller skeleton up and sit him in his lap, especially when the music gets louder and the bar is crowded with late night party fanatics and it just feels good to have Sans this close again, and if Sans minds being pressed against Gaster, held close with Gaster peppering his mandible and skull with kisses, he certainly doesn't show it. He tips his skull to the side with a sigh, small phalanges gripping at his shoulders as Gaster moves to his cervical vertebrae.

He knows the feeling of Sans' shortcuts, remembers the way they'd made him sick at first, but repetition built a tolerance. He knows Sans made one, pulled them through, and he recognizes that he's sitting with Sans in his lap in alley outside the bar, now. He shuffles them towards the back of the alley, towards a lower corner where they won't be so easily seen, and he presses Sans back against the wall, his femurs splayed beautifully in his lap.

His Sans had never made a shortcut while sitting stationary, and he wonders if could always do that or if this Sans is just particularly chaotic.

“i knew you weren't the real thing...!” Sans gasps as Gaster reaches a hand beneath his hoodie and shirt, feeling over his ribs. Gaster freezes, cocking his skull at Sans.

“Whatever do you mean, ' _I'm not the real thing'_?” He asks warily, and he tries to add a hint of offense to his tone to hide his surprise. He doesn't think he's said anything to warrant suspicion, but he could've lost himself in the moment, he decides.

Sans relaxes back against the brick behind him, pressing his chest into Gaster's hand and leveling him with an amused look. “it happens a lot, you know. at the bar, the university... stars, i've been in my own workshop at home and had W.D. Gaster show up.” He explains casually, and Gaster knows he can't hide the way his expression falls.

“they're always kind, indulgent... they listen to my theories, just like you, and often times we end up...uh, just like this...” Sans rolls his hips into Gaster's own, his magic hot and ready beneath his shorts, and Gaster idly strokes the inside of Sans ribs as he listens. “i don't know what happens to Gaster in the future, but apparently you're all really meant to screw up if i've seen this many of you, all thinking you were the only ones popping in and out of timelines.” Sans shakes his skull and huffs a laugh. “i mean i've seen you since i was a babybones...”

Gaster finds himself shook to the soul that Sans was already aware that he was lost to the timelines. It was like he'd been doused in cold water.

“the shortcut should've been a shock. it always is when you're not used to it. i've had more than enough monsters puke on me to know it takes some getting used to.”

He wants to kick himself for not realizing sooner. Then again, maybe it's better that Sans knows the difference. It rids Gaster of the guilt that Sans might've one day recounted this tryst to his own Gaster who wouldn't have known what he was talking about. “I am, admittedly, _very_ familiar with your shortcuts.” Gaster murmurs against Sans' jaw, and the smaller skeleton grins wider, if possible.

“so that means we get pretty close, huh? i mean minus when i seen you as a kid, you all seem pretty struck on me.” Sans pushes at his jacket, shoving it down Gaster's shoulders.

“I'm not telling you what will happen, Sans,” Gaster takes his jacket off, careful not to jostle Sans as he pulls it off and set it down next to him. He traces the smaller skeleton's sternum as he says, “My past may not be your future, and I shouldn't influence you to chase a path that might not be meant for you.” He shouldn't high road him like this, acting like he isn't in this situation because he made a stupid mistake, but he wants this Sans to be _happy_.

Sans snorts and gives him a challenging look. “uh, so showing up to bang me before i've even met you in my timeline is better _how_...?” And Gaster can't argue with that, but he still isn't telling Sans anything, so he busies himself with shoving his shirt up, conjuring a tongue with his magic, and laving over a couple of Sans ribs.

“I'm not telling you anything about the future, Sans.” He says with finality, and drags his tongue up Sans' sternum. Sans chokes out a gasp, and Gaster feels him rest his hand on his skull, pulling him closer.

“but you love me, right?” Sans presses, and Gaster can feel the way his soul pulses warmly as he says it. With his own Sans, he might've reached up and cupped his soul in his hands, ran delicate touches over its outer shell as he took in what Sans was feeling, but he didn't want to push this Sans too far.

He was already tangled way too far into Gaster's mess. In _many_ Gaster's messes, it seems.

“I do...” Gaster admits, leaning up to press his teeth to Sans', and he feels the way Sans' soul swells at his admission, hates that Sans feels _anything_ for him other than lust right now, hates that he's longed for him before he's ever really met him.

It's unfair to Sans to be so involved with him. It's selfish and horrible and the fact that he's done this, _different versions of him_ have done this, and have done it so regularly that Sans _loves_ him, _waits for him_. Sans knows the difference, knows that they're all alternate versions of the same person, but still chooses to love him anyway!

He hates it, hates himself as he presses his tongue into Sans' mouth when he opens for him, hates that he can't stop himself from indulging in this.

Sans hums contentedly and Gaster slides his hands down to the smaller skeleton's pelvis, stroking the arches of his iliac crests that peak out from his waistband. Sans gasps and arches his pelvis into Gaster's touch, and Gaster let's one of his hands wrap around Sans' spine and strokes up over the notches and bumps, loving the cry of pleasure he gets for it.

“Gaster...!” Sans sighs, caught between arching his spine into the hand stroking it or bucking his hips into the pinches to his crest. He settles for just writhing in Gaster's lap, magic burning hot between his femurs.

Gaster takes some pity on him, taking his hand off of his spine and reaching under the smaller skeleton's waistband to cup his magic, formed and dripping already. He presses two phalanges forward, gliding through the soft folds and gathering some of the wetness on his digits. Sans trembles in his lap, lifting his hips so Gaster has room to move his hand however he likes, which he takes full advantage of as he presses a digit _in_ , feeling how Sans clenches around him and loving how he gasps raggedly.

“You're so beautiful like this, Sans.” Gaster purrs, and Sans' hips begin moving, riding his phalange in a shaky rhythm that's definitely more his body's idea than his own, and he throws his skull back when Gaster presses a second digit in. The rolling of his hips gets a little more forceful then, more purposeful, and Sans clutches at his shirt desperately as he moves.

He decides to make it a little better for Sans and meets his thrusts, gliding his fingers in and out of his magic, and with the hand he'd been teasing Sans' crest, he shimmies the smaller skeleton's shorts down his legs. Even in the barely present light from the street lights, he can see Sans' magic flaring as he presses his phalanges into him, could see how wet he is as it dribbles down his hand.

He angles his thumb so it's pressed to Sans' clit, and it drags over the sensitive bud of magic with each thrust of his fingers, and Sans absolutely loses it. His rhythm gets hard, demanding, meeting every thrust of Gaster's phalanges, each downward movement grinding his clit against the digit presses to it and making Sans howl as he races towards his climax.

Wet sounds fill the air around them as Sans chases his end, and Gaster finds himself surprised when Sans' hands finally stop clutching his shirt desperately so he can reach up and cup his jaw, tug him down to kiss him, hard and filthy. The kiss does little to muffle Sans' wail as he comes around Gaster's digits, and Gaster takes him through it, slowing his movements gradually to draw it out as long as possible.

Sans sighs shakily, pressing their skulls together as his hips slow to a stop and just sink down tiredly onto Gaster's hand, seemingly unbothered by the digits still buried in him. Gaster is careful not to move too much and overstimulate him.

“shit, that was amazing...” Sans murmurs, frame relaxing against Gaster's, and one of his hands slips from Gaster's skull to his chest, then further down to his lap where he plucks at the fastenings of Gaster's pants. “c'mon, let me,” He urges, and Gaster presses a kiss to the top of his skull.

“You don't have to, you know.. I can take care of it.” He makes quick work of the button and zipper, and he shudders as he lets his magic finally form. Sans' hand bats his own away, stroking slowly and deliberately over his magic.

Sans tucks his skull into Gaster's shoulder as he strokes him, and Gaster forgot how good Sans could be with his hands. “uh-uh,” He tuts, twisting on the stroke just right for Gaster to gasp and buck sharply, his magic weeping over Sans' fist. “'m just getting ya warmed up.” He informs him lazily, and Gaster shudders as Sans' magic warms in his hand again. He can feel him clench against his digits, and Gaster slowly pulls them out, marveling at how wet Sans' orgasm left him.

“that's it...” Sans encourages, lifting his hips a bit and positioning them over Gaster's magic. He lines them up, and Gaster moans as he noses into the slick folds of Sans' magic. “stars, i love you so much...” Sans whispers, low and reverently like he was admitting his deepest kept secret, and he lowers his hips, sheathing Gaster in his magic.

They stay like that for a long moment, both adjusting to the intense feedback of their magic mingling and reacting to each other, and Gaster happily takes a moment to try and bury the way his soul sings for Sans, the way his chest ignites painfully at the admission. Gaster pulls him as close as he can while still giving them enough room to move, and Sans happily presses closer, hips stuttering against his own as the need to move begins to surface again.

“I'm sorry...” Gaster says with a shaky sigh, pressing his hips up into Sans and delighting in the way Sans' moans softly. He begins a slow rhythm, deep and sensual and hopefully saying all the things he couldn't. “This is so selfish...I'm sorry-” He's cut off by Sans cupping his skull in his hands again and making him look at him.

“be selfish!” Sans growls, pushing his hips down a little harder. “if i'm gonna be drug into this, than be selfish!” Sans pulls him down and kisses him hard. He growls when they part, and Gaster leans in to trail kisses over his mandible and collarbone. “you burst into my life and _made_ me love you! you just kept coming back until i couldn't help but fall for you...” He rakes his phalanges over Gaster's ribs, hard enough to mark, and Gaster deserves that... “show me how much you love _him_...”

Gaster rears back, taking in the challenge in Sans' eyelights, the overwhelming anger, the love despite it all... Sans always wore his soul on his sleeve, and Gaster sometimes forgot that. “Not him...” Gaster says as he shakes his skull. Sans gives him an aggravated look, but he just smiles back somberly. “ _You_ ,” He explains, and Sans is no more convinced, but Gaster pulls him back so he's pressed to his chest, his skull level with where Gaster's soul is brimming behind his ribs, and Sans shoots him a uneasy look.

Gaster nods at him and the smaller skeleton lets out a shaky sigh before reaching out and touching the tips of his phalanges to his soul.

He knows there's a flood of memories and raw emotions, can tell from how Sans seizes up around him as it washes over him, but he knows the most prominent is love. He knows it's not watered down, knows it's not mixed with sadness or longing.

It's just love.

This Sans is his Sans, just younger. There's nothing really different about him beyond what Gaster brought on himself, and for that he just feels love, feels happiness to _see him again_.

Sans feels it all.

He trembles in Gaster's arms, burying his face into his collarbone, but he never lets his hand leave Gaster's soul. He holds on, feeling for probably the first time in his life, and hopefully not the last, _who Gaster is_. Gaster keeps moving, keeps the same rough pace Sans had been using, and Sans sobs out a moan as he fills him. He doesn't try to stay quiet, and Gaster doesn't want him to, anyway.

“Gaster...!” He whispers, and Gaster lifts his skull so he can kiss along his jaw and skull. Tears are spilling over the edges of his sockets, the byproduct of baring their emotions so rawly no doubt. Gaster sweeps his thumb over Sans' zygomatic arch, wiping away his tears and tilting Sans' skull so he can kiss him.

They part as Gaster reaches his hand down to press his phalanges to Sans' clit, and Sans gasps sharply as they massage insistent circles over his magic. His femurs spread as wide as he can get them in their position, hips just _moving_ , trying to get Gaster's magic further inside him but trying to get more friction from his digits on his clit.

“' _Dings_!” He moans, and Gaster growls, hilts himself in Sans and releases to the gorgeous sounds of Sans crying out for him, feels him clench around him spastically as the smaller skeleton's second orgasm hits him. Gaster curls around him, holding him close as they come down.

Sans shivers as Gaster's magic dissipates, and his own is soon to follow. Gaster smooths his hands over Sans' spine, softly enough that Sans sighs against him, slumping against him tiredly.

It's quiet between them for a while, but eventually Sans finally breaks it. “i hate this part...” Sans admits softly, and Gaster reaches a hand up to tip Sans' skull up to look at him.

“What part is that?” He asks, even though he knows deep down. He hates it, too.

Sans' grin is hollow, his eyelights dim, as he murmurs sourly, “the part where you leave again.” He moves his skull out of Gaster's hand and curls into him once more, hiding. “think i'd be used to it by now...” He whispers into Gaster's collarbone, and Gaster pets over his skull soothingly.

“I'm sorry, Sans...” He says softly, sincerely, and he can feel Sans' heavy sigh against his bones more than he can hear it. He searches for anything to make him feel better. To make him happy. “...You still have your Gaster, though. In a couple of years-” He starts, but he's cut off by Sans scoffing.

“though you weren't going to influence my future?” Sans gripes, looking up at him skeptically.

He knows he could say just about anything, any excuse to make up for how bad he is at all things to do with Sans, but in the end, the only thing he can really say is, “Well, I think we've both learned that W.D. Gaster is going to be in your life, one way or another.”

Stunningly, that's what brings Sans' mood around. The smaller skeleton chuffs a laugh, his grin brightening ever so slightly. “heh, yeah...” He says somberly. “you must be pretty _bone_ -ly without me around.”

He's not sure if it's the fact that he admitted he was at fault, or if Sans just had mercy on him. He hopes it's the first option. “You have no idea.” He assures him, and Sans' grin widens when he adds, “That was terrible, by the way.”

“stick around; i've got a skele- _ton_ more!”

Gaster chuckles, leans down to kiss Sans because it's all he can bring himself to do when his chest feels like it's in a vice every time the smaller skeleton makes a joke. He hates how much he misses hearing those terrible puns regularly, hates how he wants Sans to keep going just so he'll have fresh memories for when he's tossed back into the Void, waiting for another timeline.

“y'know, when i meet him, i'm not letting him do it.” The statement is cryptic, almost random, but given everything between them and everything Gaster knows, everything Sans has learned, he knows _exactly_ what Sans is talking about, and Gaster gives him a hopeful smile.

“I hope you're the one that succeeds.” He says, and he is genuinely hopeful that _any_ Sans can keep him from being the absolute idiot he's been.

Sans grins at him reassuringly. “i know enough now that i can watch for when he gets...out of line.” He says confidently, and the flash of blue magic in his eyelight leaves no doubt in Gaster's mind that this is the Sans that will be the one to change their miserable history together, the one who will rewrite their story.

This is the Sans who doesn't care if he has to hurt Gaster to get him to see that giving up his life and happiness for that stupid CORE isn't the answer. That giving up _Sans_ isn't the answer. This Sans has determination, something whittled out of so many others Gaster has seen, even his own, and he knows it's what will change everything this time.

As he takes in the way Sans is sprawled against him, looking up at him with such hope, he knows the Gaster here is going to love him. He'll adore the challenging looks, the way Sans will pick his brain, the intense and unending way Sans will enter his life and make himself a part of it... Especially now that he knows he's meant to be at his side.

“You know what?” He says, tipping Sans' skull up to press a kiss to his teeth. “I don't think this Gaster is ready for you.” He chuckles, and Sans beams at him, a look that Gaster wants to keep in his memory forever. He hopes if he makes it home someday, that he can coax that very same look out of his own Sans.

"well, he'd better get ready," Sans insists, reaching up to press a kiss to Gaster's teeth. "'cause he's going to be mine."


End file.
